Damn, that tank is going to need one hell of a clean. The two canister filters you have mentioned are perfectly okay for your tank, what is it, 125 gallons?.The FX5 is a very good filter so there's nothing wrong with getting one, although the two filters you already have are adequate I would have thought.
I wouldn't waste your time or effort setting up that sump filter. You will however need to see how it is plumbed in. You may need to block a few holes off depending on how all the pipework has been installed.